The present invention relates to an illuminating system, particularly useful for route and destination signs and for illuminating displays, such as for aircraft, buses or other vehicles.
Readability of signs or displays, such as those for route and destination indication on public motor vehicles, buses and trains, or instrument panels for aircraft, is affected by day and night conditions, such as glare and external light. Readability can be improved by increasing contrast, since the greater the contrast, the greater will be legibility. While illumination systems have been employed to improve readability, existing illumination systems do not provide a uniform or even distribution of light. Bright spots, darkened areas at the end of the sign or display, and a halo of light along the bottom of the display or sign may appear, depending upon the arrangement of the light source utilized for illumination.
An optical film designed to convert point sources of light into an evenly dispersed area of light has been developed under the trade name 3M Brand Scotchlamp Film, and is shown in U.S. Pat. No. 4,260,220. The film is made of transparent plastic in sheet form, which is smooth on one side and grooved on the other. Its thickness is about 20 mils (0.02 inch). The grooves are actually tiny optical prisms with fragile corners and delicate faces. The film is flexible, and may be readily rolled or bent. It may be called a transreflective material.
This film is used as a part of prism light guides which distribute sunlight received from a solar light tracking system to illuminate indoor office space evenly. In the February 1987 issue of the trade magazine Architectural Lighting, there was an article about this application which suggests illuminating decorative, attention-getting displays with such prism light guides. This article suggests that colored light could be provided by fitting a luminaire with colored PAR lamps. The light could be controlled in three ways: electronic dimmers and a small microprocessor; rotating filter wheels with different colored gels; or solenoid-operated dichroic filters in front of white light sources.
Further, the use of rotatable translucent multi-color tubes is known in which a light source is arranged directly inside the tube itself and portions of the color tube are shielded so that only a desired colored segment of the color tube is illuminated by the light source. An example of such a tube is disclosed in U.S. Pat. No. 2,374,640.